Developments with Metallic Thermal Interface Materials

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Reliability of electronic modules and systems is critical. For decades, the need for temperature modulation and control has been identified as a principal factor impacting semiconductor and packaging reliability. In most electronic systems, individual semiconductors are designed, manufactured, and sold for application within a system manufacturer’s product. The interface between the external mounting surface of the semiconductor package and any required thermal management component is increasingly the center of attention as efforts continue toward improving the performance and reliability of the overall system.

Thermal Interface Material Function

Thermal interface materials (TIM) provide a critical function on the external surface of the module or device and within a semiconductor package, such as a high-performance server processor module where several semiconductor die and one or more heat spreaders or a module lid are employed to provide the most effective heat transfer possible. The critical role of the thermal interface material is to improve the efficiency of heat transfer from the external mating surface of the semiconductor device and the surface to which it is attached, typically an air-cooled heat sink, liquid cold plate, or the metal surface of some other component.

Application Interface Conditions and Impact on Thermal Performance

The ideal interface consists of metal-to-metal contact across the contact area, which would require precision machining and polishing of the two surfaces to a degree and also add significant manufacturing costs to those components. In lieu of a set of ideal polished surfaces, the efficient TIM provides a very thin thermally conducting material which, given variation in metal surfaces, may vary in thickness through the interface. The thickness of the metal TIM at various points across the interface would be determined by several factors: the type of mechanical fasteners used to attach the device to the heat sink or cold plate, the amount of clamping force exerted, the location of the fasteners, and the degree of roughness and flatness of the two manufactured surfaces. The surface of a liquid cold plate, for example, may be a machined surface of a casting (which may expose internal voiding within the casting), the machined surface of an aluminum or copper cold plate, or the raw extruded surface of an aluminum cold plate, if no machining is specified. The mating surface of the heat sink or cold plate may also have variations due to warpage or bending (depending on the thickness), the care exercised when handling during manufacturing and assembly, and the relative clamping force applied versus the stiffness and strength of the heat sink or cold plate.

If mechanical fasteners such as screws or bolts are located only at the periphery of a large module, the flatness of the module metal baseplate can be altered as fasteners are torqued into place. This can change the physical characteristics of the interface when measured at a greater distance from the locations of the fasteners. For instance, standard power semiconductor modules, known as isolated gate bipolar transistors (IGBTs), are very common components used in electrical drives and machine tools, controlling wing flaps and actuators for aircraft, and switching devices within electrical inverters for propulsion powertrains in vehicles. Standard IGBT module footprints have industry-standard dimensions, with specified locations for fasteners, which are typically at the periphery of the device. There are also some industry designs for small modules, which include one or more fasteners in the center of the device.

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Editor's Note: This article originally appeared in the March issue of SMT Magazine.